Video-on-demand system capable of performing a high-speed playback at a correct speed

ABSTRACT

A video server apparatus is provided with a stream control section which determines a keyframe readout interval and a keyframe playback interval that satisfy a playback speed designated by a terminal apparatus. The video server apparatus transmits data of the thus-determined keyframe playback interval to the terminal apparatus. The video server apparatus reads out keyframes of a stored bit stream from at the thus-determined keyframe readout interval, and transmits a stream including the read-out keyframes to the terminal apparatus. The terminal apparatus decodes and displays the transmitted stream at the received keyframe playback interval. This system can correctly perform a playback at an arbitrary speed that is designated by the terminal apparatus.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a video server apparatus capable ofperforming various kinds of special playbacks of coded moving imagesmore smoothly.

The invention also relates to a video-on-demand system, a broadcastingsystem, a video library system, and the like which consist of a videoserver apparatus for storing moving image programs and terminalapparatuses for displaying images supplied from the video server, andparticularly to a technique for enabling a high-speed playback of movingimages at an arbitrary speed.

2. Description of the Related Art

A video server apparatus has entered a commercial stage which apparatusstores a number of compressed/coded moving image programs in a storagemedium such as a hard disk and performs simultaneous, multiple playbacksof the same or different image programs in response to requests from aplurality of terminal apparatuses. A video-on-demand system has beenrealized by combining the above server apparatus and terminalapparatuses in which system the server apparatus immediately responds toindividual program requests from terminal apparatuses. In museums etc.,a video library system utilizing such a video-on-demand system isconstructed to allow visitors to conduct in-house perusal. Further, CATVbroadcasting system companies are now considering a service forproviding, by utilizing a video-on-demand system, a movie that isselected by a user at his terminal apparatus. In this specification, allof the above systems are generally referred to as a "video-on-demandsystem."

In the above systems, not only a normal playback but also various kindsof special playbacks such as a fast-forward playback and a rewindplayback are attracting great interest. For example, reference is madeto Y. Ishibashi et al.: "Multiple-access Visual Search for DigitalVideo-on-demand Services," technical Report of IEICE, CS92-74, IE92-96,pp. 101-106, December 1992, and T. Mori et al.: "Video Storage andVisual Search techniques in VOD System," Technical Report of IEICE,IE94-81, pp. 9-16, November 1994.

A conventional video-on-demand system will be described below. FIG. 1 isa block diagram showing its configuration. In FIG. 1, a video serverapparatus VS consists of a hard disk 1 as a storage medium, AV(audio-video) streams 1a, an access table 1b, a stream reading section2; a stream transmitting section 3; an access table managing section 4;a command receiving section 5, and a stream control section 6. Aterminal apparatus ST consists of a stream receiving section 7, acommand transmitting section 8, a decoder section 9, and an inputcontrol section 10. A monitor 11, a speaker 12, and an input device 13are connected to the terminal apparatus ST.

FIG. 2 is a block diagram showing details of the stream control section6 of the above conventional video server apparatus VS. In FIG. 2,reference numeral 81 denotes a stream readout position determiningsection and numeral 82 denotes a keyframe skip number setting table.

FIG. 3A illustrates keyframe readout intervals of a high-speed playbackin the above server apparatus. In FIG. 3A, reference numerals 41-43denote keyframes that are read out first, second, and third, andcharacters a-c denote readout periods of the keyframes 41-43 (constantread bit rate).

FIG. 4A illustrates keyframe readout intervals of another type ofhigh-speed playback in the above server apparatus. In FIG. 4A, referencecharacter n means time taken to read out a storage unit block at aconstant bit rate. Reference numerals 51-53 denote keyframes that areread out first, second, and third, and symbols a'-c' denote readoutperiods of the keyframes 51-53 when they are read out at a constant bitrate on a storage unit block basis.

FIG. 5A illustrates the operation of the access table managing section 4of the above server apparatus. In FIG. 5A, reference character u meanstime taken to read out the access table 1b at a constant bit rate.

FIG. 6A illustrates a frame storage order of an AV stream 1a, and FIG.6B illustrates a frame playback order in a normal playback.

The above-configured video server apparatus operates in the followingmanner. A signal that is input from the input device 13 is converted bythe input control section 10 into commands indicating a playback startposition and a playback speed, which are transmitted from the commandtransmitting section 8 to the command receiving section 5. The streamcontrol section 6 determines keyframes to be read out and their storagelocations based on the playback start position and the playback speedthat are received by the command receiving section 5 and keyframeaddresses that are managed by the access table managing section 4, andgives the determined information to the stream reading section 2. Morespecifically, in the stream control section 6, the stream readoutposition determining section 81 determines keyframes to be read outbased on the playback speed that is obtained from the command receivingsection 5, the keyframe interval, and the coding bit rate of the AVstream 1a, and also determines readout positions of the AV stream 1abased on the playback start position that is obtained from the commandreceiving section 5 and keyframe addresses that are managed by theaccess table managing section 4. The determined information is sent tothe stream reading section 2. The stream reading section 2 reads out theAV stream 1a at a designated read bit rate, and a read-out stream istransmitted from the stream transmitting section 3 to the streamreceiving section 7. The stream received by the stream receiving section7 is decoded by the decoder section 9, and output as images and soundfrom the monitor 11 and the speaker 12, respectively.

An AV stream 1a, which has been obtained by compressing/coding anaudio-associated video signal according to the international standardMPEG (moving picture experts group), are stored in a frame order shownin FIG. 6A. In the case of a normal playback, the stream reading section2 reads out all of the AV stream 1a, the stream transmitting section 3transmits all of the read-out data, and the data are output from themonitor 11 in a frame order shown in FIG. 6B. In the case of afast-forward or rewind playback, the stream reading section 2 reads outonly keyframes, i.e., I frames, the stream transmitting section 3transmits only the keyframes, and the keyframes are output from themonitor 11 in a frame order of FIG. 3A, FIG. 4A, or the like.

For convenience of description, frame types of I, P and B and displayframe order numbers of a normal playback are indicated in FIGS. 3A, 4Aand 6A. Frame type I corresponds to an intra-coded image that has beensubjected to intraframe coding, frame type P corresponds to a forwardprediction coded image that has been subjected to motion-compensationinterframe coding, and frame type B corresponds to a bidirectionalprediction coded image that has been subjected to motion-compensationinterframe coding.

The access table 1b has descriptions of a head address of each I frameof an AV stream 1a, a data length of each I frame, a coding bit rate ofthe AV stream 1a, and other items, and is managed by the access tablemanaging section 4.

However, the above conventional video server apparatus has a problemthat where the data length of a keyframe varies stream by stream, aconstant playback speed is not obtained in reading streams at a constantbit rate in a high-speed playback even if the same keyframe skip numberis set.

The above conventional video server apparatus has another problem thatwhere the data length of a keyframe varies in one stream, the time takento read or transmit one keyframe varies in reading or transmitting astream at a constant bit rate in a high-speed playback.

The above conventional video server apparatus has still another problemthat where the data length of a keyframe varies in one stream, the timetaken to read or transmit one keyframe varies in reading or transmittinga stream at a constant bit rate on a fixed-length block basis in ahigh-speed playback.

Further, in the above conventional video server apparatus, since theentire access table is acquired immediately before a playback at aconstant bit rate, a response time from a stream playback request to astart of an actual playback of the requested stream is long.

FIG. 7 shows another conventional video-on-demand system. As shown inFIG. 7, a video server apparatus of this system consists of a hard disk101 for storing a number of AV streams 101a, a hard disk 102 for storingan access table 102b to be used in accessing the AV streams 101a, astream reading section 103 for reading out an AV stream 101a from thehard disk 101, a stream transmitting section 104 for transmitting thereadout AV stream to a terminal apparatus, an access table managingsection 105 for managing the access table 102b, a command receivingsection 106 for receiving a command from the terminal apparatus, astream control section 107 for controlling reading and transmission ofthe AV stream in response to a request from the terminal apparatus, anda keyframe skip number setting table 108 in which keyframe readoutintervals of high-speed playbacks are set.

The terminal apparatus consists of an input control section 113 forgenerating a command in response to an input manipulation on an inputdevice 116, a command transmitting section 109 for transmitting thegenerated command to the video server apparatus, a stream receivingsection 110 for receiving an AV stream 101a from the video serverapparatus, a decoder section 112 for decoding the received AV stream101a, and a decoder control section 111 for controlling the decoding andplayback timing of the decoder section 112. The input device 116 such asa keyboard or a mouse, a monitor 114 for displaying decoded images and aspeaker 115 for outputting decoded sound are connected to the terminalapparatus.

In this video-on-demand system, a signal that is input through the inputdevice 116 and indicates a designated program, a playback startposition, a playback speed, etc. is converted by the input controlsection 113 into a program designation command and commands indicating aplayback start position and a playback speed of moving images, whichcommands are transmitted from the command transmitting section 109 tothe command receiving section 106 of the video server apparatus.

The stream control section 107 selects an AV stream 101a of thedesignated program from among a number of AV streams 101a stored in thehard disk 101 and determines the storage location of a keyframe locatedin the vicinity of the designated playback start position based on theprogram designation information, the playback start position, and theplayback speed that have been received by the command receiving section106, and keyframe information and a coding bit rate of the AV stream101a which are managed by the access table managing section 105. Thestream control section 107 further determines a read bit rate based onthe coding bit rate etc. The stream control section 107 gives thethus-determined information to the stream reading section 103 andinstructs it to start reading out the selected stream 101a.

In a normal playback, the stream control section 107 sets the read bitrate equal to the coding bit rate. In a high-speed playback, the streamcontrol section 107 sets the read bit rate equal to or lower than thecoding bit rate.

If the playback speed obtained from the command receiving section 106means a high-speed playback, the stream control section 107 determineskeyframes to be read out in the high-speed playback and a read bit ratebased on the received playback speed, the relationship between theplayback speed multiplication factor and the keyframe readout intervalwhich is described in the keyframe skip number setting table 108, andthe information managed by the access table managing section 105. Thestream control section 107 then instructs the stream reading section 103to read out the keyframes thus determined, and to add a playback modeindicating a change in playback speed to a read-out stream.

The stream reading section 103 reads out the designated keyframes fromthe AV stream 101a at the read bit rate that is designated by the streamcontrol section 107, and inserts or buries a playback mode into theread-out stream that consists only of the keyframes.

The stream transmitting section 104 transmits the stream that isreceived from the stream reading section 103 to the stream receivingsection 110 of the terminal apparatus. At this time, the stream controlsection 107 controls the transmission bit rate in accordance with thenumber of terminal apparatuses currently communicating with the videoserver apparatus and other information.

In the terminal apparatus, the decoder control section 111 controls thedecoding and playback timing of the decoder section 112 by using, as atrigger, the playback mode extracted from the stream by the streamreceiving section 110. Controlled in this manner, the decoder section112 decodes the stream received by the stream receiving section 110. Thedecoded stream is output from the monitor 114 as images, and also outputfrom the speaker 115 as sound in the case of a normal playback.

In the case of a normal playback, the entire AV stream 101a that isstored in the hard disk 101 in a frame order shown in FIG. 6A is readout by the stream reading section 103, and then transmitted by thestream transmitting section 104. The stream 101a is output from themonitor 114 in a frame order shown in FIG. 6B.

In the case of a high-speed playback, the stream reading section 103reads out only the designated keyframes (i.e., I frames), the streamtransmitting section 104 transmits the read-out keyframes, and themonitor 114 continues to display each of the transmitted I frames over agiven period as shown in FIGS. 8A-8D. In FIGS. 8A-8D, solid-line framesare frames that are actually decoded and broken-line frames are framesthat are a previously decoded frame which is kept displayed as it is.FIG. 8A shows a case where I frames are read out in the order of I(1),I(16), I(31), I(46), . . . (the parenthesized numbers indicate a displayorder of frames in a normal playback) and each of the I frames isdisplayed on the monitor 114 over a playback period of 9 frames.Similarly, FIG. 8B shows a case where I frames are read out in the orderof I(1), I(31), I(61), I(91), . . . and each of the I frames isdisplayed on the monitor 114 over a playback period of 9 frames.

In the case of FIG. 8A, images that would have a frame interval of 15frames in a normal playback are played back over a frame interval of 9frames (images are updated every 9 frames). Therefore, the playbackspeed multiplication factor is 15/9=1.67. In the case of FIG. 8B, imagesthat would have a frame interval of 30 frames in a normal playback areplayed back over a frame interval of 9 frames. Therefore, the playbackspeed multiplication factor is 30/9=3.33.

FIG. 9 is a playback speed table showing a relationship between aplayback speed multiplication factor and a combination of a keyframereadout interval and a keyframe playback interval. This table relates toa case where the keyframe interval, which is the number of framesexisting between adjacent keyframes in a stream, is 15. The horizontalaxis represents a keyframe readout interval x, that is, a frame intervalbetween actually read-out keyframes of an AV stream 101a, and thevertical axis represents a keyframe playback interval y, that is, aframe interval during which the terminal apparatus continues to playback the same keyframe. A playback speed, which is calculated as x/y, iswritten at an intersection of x and y.

In the conventional video server apparatus, the keyframe skip numbersetting table 108 describes, in accordance with the systemspecification, a relationship corresponding to part of the playbackspeed table of FIG. 9, for instance, a relationship between the framereadout interval x and the playback speed multiplication number in acase where the keyframe playback interval is 9 (region 191 in FIG. 9).

When a playback speed of a multiplication factor 2, 3, 4 or 5 isdesignated by the terminal apparatus, the stream control section 107 ofthe video server apparatus selects a playback speed that is closest tothe designated speed by referring to the relationship described in thekeyframe skip number setting table 108, and determines a keyframereadout interval x that corresponds to the selected playback speed. Inthis example, when a double-speed playback is designated by the terminalapparatus, a keyframe readout interval x of 15 corresponding to aplayback speed multiplication factor 1.67 is selected. When atriple-speed playback is designated, a keyframe readout interval x of 30corresponding to a multiplication factor 3.33 is selected. When aquadruple-speed playback is designated, a keyframe readout interval x of30 corresponding to a multiplication factor 3.33 is also selected. Whena 5-fold-speed playback is designated, a keyframe readout interval x of45 corresponding to a multiplication factor 5 is selected. The streamreading section 103 is so controlled as to read out I frames of the AVstream 101a from the hard disk 101 at the keyframe readout interval xthus determined. In this example, the actual playback speedmultiplication factors are 1.67, 3.33, 3.33 and 5 for the designatedmultiplication factors of 2, 3, 4 and 5, respectively.

According to the system specification, the terminal apparatus continuesto display each keyframe that is transmitted from the video serverapparatus while setting the keyframe playback interval y at 9. FIGS.8A-8D show playback frame sequences in the terminal apparatus fordesignated playback speed multiplication factors of 2, 3, 4 and 5,respectively.

However, in the conventional video-on-demand system, even if anarbitrary playback speed multiplication factor is designated by theterminal apparatus, an actual playback is performed with amultiplication factor that is an approximation of the designated one.Further, there may occur an event that actual playbacks are performed atthe same speed even for designated playback speeds that are differentfrom each other, as in the case of triple-speed and quadruple-speedplaybacks in the above example. As such, the actual playback speedcannot be controlled finely.

Further, there is a possibility that because the keyframe readoutinterval varies stream by stream, a designated playback speed cannot beattained without changing the keyframe playback interval. However, inthis case, since the conventional system does not have any means forinforming the terminal apparatus of a keyframe playback interval thathas been determined for reasons on the video server apparatus side, theterminal apparatus cannot control the keyframe playback interval inaccordance with the characteristics of a stream. As a result, ahigh-speed playback is performed at an improper speed.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a video serverapparatus which can perform a high-speed playback at a playback speedrequested from a terminal apparatus even if the keyframe data sizevaries stream by stream.

A second object of the invention is to provide a video server apparatuswhich can transmit frames at constant intervals in a high-speed playbackeven if the keyframe data size varies in one stream.

A third object of the invention is to provide a video server apparatuswhich can shorten a response time from a playback request from aterminal apparatus to a start of transmission of a stream irrespectiveof the size of an access table.

A fourth object of the invention is to provide a video-on-demand systemwhich can perform a high-speed playback at a correct, arbitrary speed,as well as to provide a video server apparatus and a terminal apparatusconstituting such a system.

According to a first aspect of the invention, there is provided a videoserver apparatus comprising stream storage means for storing compressedand coded streams; stream reading means for reading out one of thestreams which is designated by a terminal apparatus; stream transmittingmeans for transmitting the read-out stream to the terminal apparatus;access table managing means for managing addresses of keyframes of thestream; keyframe data length calculating means for acquiring datalengths of the keyframes; and stream readout position determining meansfor determining a first keyframe located in the vicinity of a playbackstart position that is designated by the terminal apparatus based on theaddresses managed by the access table managing means, informing thestream reading means of a readout position of the first keyframe,selecting keyframes to be extracted from the stream so as to satisfy anarbitrary playback speed that is designated by the terminal apparatusbased on a bit rate at which the stream reading means reads out thestream, a coding bit rate and a keyframe interval of the stream, and thekeyframe data lengths acquired by the keyframe data length calculatingmeans, and informing the stream reading means of the selected keyframes.With this configuration, even if the keyframe data length varies streamby stream, a high-speed playback can be performed smoothly at a speedthat is closest to an arbitrary playback speed requested by the terminalapparatus with a minimized variation in playback speed among streams.

Further, there is provided a video server apparatus comprising streamstorage means for storing compressed and coded streams; stream readingmeans for reading out one of the streams which is designated by aterminal apparatus; stream transmitting means for transmitting theread-out stream to the terminal apparatus; access table managing meansfor managing addresses of keyframes of the stream; keyframe data lengthcalculating means for acquiring data lengths of the keyframes; streamreadout position determining means for determining a first keyframelocated in the vicinity of a playback start position that is designatedby the terminal apparatus based on the addresses managed by the accesstable managing means, selecting keyframes which satisfy an arbitraryplayback speed that is designated by the terminal apparatus, andinforming the stream reading means of readout positions of the firstkeyframe and the selected keyframes; and stream transmission timingsetting means for controlling a transmission bit rate of the streamtransmitting means and a read bit rate of the stream reading means sothat the selected keyframes are transmitted at a constant interval,based on the keyframe data lengths acquired by the keyframe data lengthcalculating means. With this configuration, even if the keyframe datalength varies within one stream, keyframes are transmitted to theterminal apparatus at a constant interval in a high-speed playback of adesignated, arbitrary speed. This enables a high-speed playback to beviewed more comfortably.

Still further, there is provided a video server apparatus comprisingstream storage means for storing compressed and coded streams; accesstable storage means for storing an access table having pages whichcontain addresses of keyframes of the streams, each of the pagescorresponding to an arbitrary data length or an arbitrary period; streamreading means for reading out one of the streams which is designated bya terminal apparatus; stream transmitting means for transmitting theread-out stream to the terminal apparatus; keyframe data lengthcalculating means for acquiring data lengths of the keyframes; accesstable managing means for acquiring addresses of the keyframes from theaccess table storage means on a page-by-page basis; stream readoutposition determining means for determining a first keyframe located inthe vicinity of a playback start position that is designated by theterminal apparatus based on the addresses acquired by the access tablemanaging means, and informing the stream reading means of a readoutposition of the first keyframe; and access table page switching judgmentmeans for judging whether to acquire a next page based on an address ofa keyframe being read out by the stream reading means, and forinstructing the access table managing means to acquire the next pagebased on a judgment result. With this configuration, a playback can bestarted before acquisition of addresses of all the keyframes. Therefore,a response time from a stream playback request to an actual start of aplayback can be shortened.

According to a second aspect of the invention, there is provided avideo-on demand system comprising a video server apparatus for storingbit streams; a terminal apparatus for requesting the video serverapparatus to transmit one of the bit streams; stream control means fordetermining, when only keyframes are read out from the bit stream inaccordance with a playback speed designated by the terminal apparatus, akeyframe readout interval and a keyframe playback interval that satisfythe designated playback speed; means provided in the video serverapparatus, for reading out the keyframes of the bit stream at thekeyframe readout interval, and for transmitting a stream including theread-out keyframes to the terminal apparatus; and means provided in theterminal apparatus, for decoding and displaying the transmitted streamat the keyframe playback interval. This system can perform a playback ata correct speed that is equal to an arbitrary speed designated by theterminal apparatus.

The above video-on-demand system may be constructed such that the streamcontrol means is provided in the video server apparatus, and that dataof the keyframe playback interval determined by the stream control meansis transmitted to the terminal apparatus.

The above video-on-demand system may further comprise controlinformation transmitting and receiving means for sending the data of thekeyframe playback interval from the video server apparatus to theterminal apparatus. In this configuration, data of the keyframe playbackinterval and a read-out stream are transmitted via separate routes.

The above video-on-demand system may further comprise means provided inthe video server apparatus, for inserting the data of the keyframeplayback interval into the stream to be transmitted to the terminalapparatus. This configuration allows the terminal apparatus to easilyperform precise control of the keyframe playback interval.

The above video-on-demand system may be constructed such that the streamcontrol means is provided in the terminal apparatus, and that data ofthe keyframe readout interval is transmitted to the video serverapparatus. This configuration allows the terminal apparatus to performprecise control of the keyframe playback interval.

The above video-on-demand system may be constructed such that the streamcontrol means determines employs, as the keyframe playback interval, ashortest one of integral multiples of a keyframe interval of the bitstream which are longer than a minimum keyframe playback interval thatallows complete transmission of the keyframes, and employs, as thekeyframe readout interval, a product of the keyframe playback intervaland a multiplication factor of the designated playback speed.

The above video-on-demand system may be constructed such that the streamcontrol means determines sets, as a basic keyframe playback interval, ashortest one of integral multiples of a keyframe interval of the bitstream which are longer than a minimum keyframe playback interval thatallows complete transmission of the keyframes, employs, as the keyframeplayback interval, a shortest one of intervals from the minimum keyframeplayback interval to the basic keyframe playback interval, and employs,as the keyframe readout interval, a product of the keyframe playbackinterval and a multiplication factor of the designated playback speed.With this configuration, the quality of high-speed playback images canbe improved by making the keyframe playback interval as short aspossible.

The above video-on-demand system may be constructed such that the streamcontrol means employs, as the keyframe readout interval, a shortest oneof integral multiples of a keyframe interval of the bit stream which arelonger than a product of a multiplication factor of the designatedplayback speed and a minimum keyframe playback interval that allowscomplete transmission of the keyframes, and employs, as a keyframeplayback interval of an nth keyframe received by the terminal apparatus,an integer that is obtained by subtracting an integer that is closest toa value obtained by multiplying the keyframe readout interval by n-1 andthen dividing a resulting product by the multiplication factor from avalue obtained by multiplying the keyframe readout interval by n andthen dividing a resulting product by the multiplication factor. Withthis configuration, a non-integral playback speed multiplication factorcan be attained because the keyframe playback interval is determined foreach keyframe.

The above video-on-demand system may be constructed such that the streamcontrol means determines the keyframe readout interval and the keyframeplayback interval that satisfy the designated playback speed of aforward or backward high-speed playback. With this configuration, ahigh-speed playback can be performed at a correct speed in eitherforward or backward direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the configuration of a conventionalvideo-on-demand system including a video server apparatus;

FIG. 2 is a block diagram showing the configuration of a stream controlsection of the conventional video server apparatus of FIG. 1;

FIG. 3A illustrates keyframe readout intervals in a first-type ofhigh-speed playback in the conventional video server apparatus of FIG.1;

FIG. 3B illustrate keyframe readout intervals in the first type ofhigh-speed playback in a video server apparatus according to a secondembodiment of the present invention;

FIG. 4A illustrates keyframe readout intervals in a second-type ofhigh-speed playback in the conventional video server apparatus of FIG.1;

FIG. 4B illustrate keyframe readout intervals in the second type ofhigh-speed playback in a video server apparatus according to a secondembodiment of the invention;

FIG. 5A illustrates the operation of an access table managing section ofthe conventional video server apparatus of FIG. 1;

FIG. 5B illustrates the operation of an access table managing section ofa video server apparatus according to a third embodiment of theinvention;

FIG. 6A illustrates a frame storage order of an AV stream;

FIG. 6B illustrates a frame playback order of an AV stream in a normalplayback;

FIG. 7 is a block diagram showing the configuration of anotherconventional video-on-demand system;

FIGS. 8A-8D illustrate playback fame sequences in the conventionalsystem of FIG. 7;

FIG. 9 is a playback speed table illustrating how a keyframe readoutinterval and a keyframe playback interval are determined in theconventional system of FIG. 7;

FIG. 10 is a block diagram showing the configuration of avideo-on-demand system including a video server apparatus according to afirst embodiment of the present invention;

FIG. 11 is a block diagram showing the configuration of a stream controlsection of the video server apparatus of the first embodiment;

FIG. 12 is a block diagram showing the configuration of a stream controlsection of a video server apparatus according to the second embodimentof the invention;

FIG. 13 is a block diagram showing the configuration of a stream controlsection of a video server apparatus according to the third embodiment ofthe invention;

FIG. 14 is a block diagram showing a video-on-demand system according toa fourth embodiment of the invention;

FIG. 15 is a playback speed table illustrating how a keyframe readoutinterval and a keyframe playback interval are determined in avideo-on-demand-system according to a fifth embodiment of the invention;

FIGS. 16A-16D illustrate playback frame sequences in thevideo-on-demand-system of the fifth embodiment;

FIG. 17 is a playback speed table illustrating how a keyframe readoutinterval and a keyframe playback interval are determined in avideo-on-demand-system according to a sixth embodiment of the invention;

FIGS. 18A-18D illustrate playback frame sequences in thevideo-on-demand-system of the sixth embodiment;

FIG. 19 is a playback speed table illustrating how a keyframe readoutinterval and a keyframe playback interval are determined in avideo-on-demand-system according to a seventh embodiment of theinvention;

FIGS. 20A-20D illustrate playback frame sequences in thevideo-on-demand-system of the seventh embodiment;

FIG. 21 is a block diagram showing a video-on-demand system according toan eighth embodiment of the invention; and

FIG. 22 is a block diagram showing a video-on-demand system according toa ninth embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be hereinafter described withreference to the accompanying drawings.

EMBODIMENT 1

FIG. 10 shows the configuration of a video-on-demand system including avideo server apparatus according to a first embodiment of the invention.In FIG. 10, a video server apparatus VS consists of a hard disk 1 as astorage medium, an AV stream 1a, an access table 1b, a stream readingsection 2; a stream transmitting section 3; an access table managingsection 4; a command receiving section 5, and a stream control section60. A terminal apparatus ST consists of a stream receiving section 7, acommand transmitting section 8, a decoder section 9, and an inputcontrol section 10. A monitor 11, a speaker 12, and an input device 13are connected to the terminal apparatus ST. The video server apparatusof this embodiment different from the conventional video serverapparatus of FIG. 1 only in the stream control section 60.

The video server apparatus VS of this embodiment will be describedbelow. FIG. 11 shows the configuration of the stream control section 60of the video server VS. As shown in FIG. 11, the stream control section60 has a stream reading position determining section 21 for determininga keyframe that is located in the vicinity of a playback start positiondesignated by the terminal apparatus ST while cooperating with theaccess table managing section 4, and for informing the stream readingsection 2 of a readout position of the determined keyframe. The streamcontrol section 60 also has a keyframe data length calculating section22 for acquiring data lengths of keyframes of arbitrary intervals fromthe access table managing section 4.

The operation of the above-configured video server apparatus VS will bedescribed below (the stream control section 60 will be mainlydescribed). When the command receiving section 5 has received a playbackstart position, the stream readout position determining section 21determines a readout position of a keyframe that is located in thevicinity of the designated playback start position based on the receivedplayback start position and an address of the keyframe which is acquiredby the access table managing section 4 from the access table 1b that isstored in the hard disk 1. When the command receiving section 5 has notreceived any playback start position, the stream readout positiondetermining section 21 determines a readout position of a keyframe thatis designated to be read out next or continuous readout positions of theAV stream 1a. The determined information is sent to the stream readingsection 2.

If a playback speed received by the command receiving section 5 means adiscontinuous playback such as a high-speed playback, the stream readoutposition determining section 21 calculates time that is necessary toread out a keyframe having an average data length based on all or anarbitrary number of keyframe data lengths acquired from the access table1b and a bit rate at which the stream read section 2 reads out the AVstream 1a, and then assumes a keyframe transmission interval. The streamreadout position determining section 21 then determines a keyframe to beread out next that satisfies the designated playback speed based on theassumed keyframe transmission interval, the keyframe interval, and theframe frequency. If a playback speed received by the command receivingsection 5 means a continuous playback such as a normal playback, a slowplayback, or the like, the stream readout position determining section21 does not determine a keyframe to be read out next.

Specifically, if a playback speed received by the command receivingsection 5 means a discontinuous playback such as a high-speed playback,a keyframe average data length KL is calculated as

    KL=(A1+A2+ . . . +Aa)/a                                    (1)

where a is the number of keyframe data acquired from the access table1b, and Ai (i=1, 2, . . . , a) is each keyframe data length.

Further, the time KT necessary to read out a keyframe having the averagedata length KL and a playback speed PS are calculated as

    KT=KL/BR                                                   (2)

    PS=(KI/FR)(NS+1)/KT                                        (3)

where BR is a bit rate at which the stream reading section 2 reads outthe AV stream 1a, KI is the keyframe interval, FR is the framefrequency, and NS is a keyframe skip number to a keyframe to be read outnext.

By using the above equations, the keyframe skip number NS is sodetermined that the playback speed PS becomes closest to the designatedplayback speed. If the keyframe interval KI is not constant, an averageof keyframe intervals of a stream, for instance, is employed as KI.

The stream reading section 2 reads out the keyframes of the AV stream 1awhich are designated by the stream readout position determining section21 or frames at continuous readout positions, sends those frames to thestream transmitting section 3. The stream transmitting section 3transmits a stream to the terminal apparatus ST at constant intervals.

According to the first embodiment, by providing the keyframe data lengthcalculating section 22 in the stream control section 60, a keyframe tobe read out next is so determined as to keep a playback speed of ahigh-speed playback based on keyframe data lengths. Therefore, even ifthe keyframe data length varies stream by stream, a playback speedclosest to a designated playback speed is attained and hence a variationin playback speed among streams is minimized, whereby a high-speedplayback can be performed more smoothly.

EMBODIMENT 2

A video server apparatus according to a second embodiment of theinvention will be described below. FIG. 12 shows the configuration of astream control section 60a of a video server apparatus VS of thisembodiment. The other part of the video server apparatus VS is the sameas that of the first embodiment.

As shown in FIG. 12, the stream control section 60a has a stream readingposition determining section 31 for determining a keyframe that islocated in the vicinity of a playback start position designated by theterminal apparatus ST while cooperating with the access table managingsection 4, selecting keyframes that satisfy a playback speed designatedby the terminal apparatus ST, and informing the stream reading section 2of readout positions of those keyframes. The stream control section 60aalso has a keyframe data length calculating section 32 for acquiringkeyframe data lengths of arbitrary intervals from the access tablemanaging section 4, and a stream transmission timing setting section 33for informing the stream transmitting section 3 of transmission timingof each keyframe.

FIG. 3B illustrates keyframe readout intervals of a first type ofhigh-speed playback in this embodiment. In FIG. 3B, character mrepresents time that is taken to read out a maximum keyframe of a streamat a constant bit rate. FIG. 4B illustrates keyframe readout intervalsof a second type of high-speed playback in this embodiment. In FIG. 4B,symbol n' represents time that is taken to read out, at a constant bitrate, most storage unit blocks encompassed by a keyframe of a stream.The other symbols in FIGS. 3B and 4B represent the same items as inFIGS. 3A and 4A, which correspond to the conventional video serverapparatus.

The operation of the above-configured video server apparatus VS will bedescribed below (the stream control section 60a will be mainlydescribed). When the command receiving section 5 has received a playbackstart position, the stream readout position determining section 31determines a readout position of a keyframe that is located in thevicinity of the designated playback start position based on the receivedplayback start position and an address of the keyframe which is acquiredby the access table managing section 4 from the access table 1b that isstored in the hard disk 1. When the command receiving section 5 has notreceived any playback start position, the stream readout positiondetermining section 31 determines a readout position of a keyframe thatis designated to be read out next or continuous readout positions of theAV stream 1a. The determined information is sent to the stream readingsection 2.

If a playback speed received by the command receiving section 5 means adiscontinuous playback such as a high-speed playback, the stream readoutposition determining section 31 calculates a readout time m of themaximum keyframe based on all or an arbitrary number of keyframe datalengths acquired from the access table 1b and the maximum bit rate atwhich the stream read section 2 reads out the AV stream 1a, employs thecalculated readout time m as a keyframe transmission interval, andinforms the stream transmission timing setting section 33 of thecalculated keyframe transmission interval. The stream readout positiondetermining section 31 then determines a keyframe to be read out nextthat satisfies the designated playback speed based on the calculatedkeyframe transmission interval, the keyframe interval, and the framefrequency. If a playback speed received by the command receiving section5 means a continuous playback such as a normal playback, a slowplayback, or the like, the stream readout position determining section31 does not determine a keyframe to be read out next.

Specifically, if a playback speed received by the command receivingsection 5 means a discontinuous playback such as a high-speed playback,the readout time m of the maximum keyframe is calculated as

    m=Am/BR                                                    (4)

where Am is the maximum keyframe data length acquired from the accesstable 1b and BR is a bit rate at which the stream reading section 2reads out the AV stream 1a.

Further, a playback speed PS is calculated as

    PS=(KI/FR)(NS+1)/m                                         (5)

where KI is the keyframe interval, FR is the frame frequency, and NS isa keyframe skip number to a keyframe to be read out next.

By using the above equations, the keyframe skip number NS is sodetermined that the playback speed PS becomes closest to the designatedplayback speed. If the keyframe interval KI is not constant, an averageof keyframe intervals of a stream, for instance, is employed as KI.

In the case of a high-speed playback, the stream transmission timingsetting section 33 instructs the stream transmitting section 3 totransmit a keyframe as read out by the stream reading section 2 at thekeyframe transmission interval that is designated by the stream readoutposition determining section 31, and informs the stream reading section2 of read timing of the next keyframe.

Next, a description will be made of a case where the stream readingsection 2 reads out an AV stream 1a on an arbitrary, fixed-length blockbasis. If a playback speed received by the command receiving section 5means a discontinuous playback such as a high-speed playback, the streamreadout position determining section 31 calculates a readout time n' ofa maximum number of fixed-length blocks of the AV stream 1a encompassedby a keyframe based on the received playback speed, all or an arbitrarynumber of keyframe addresses and data lengths acquired from the accesstable 1b by the keyframe data length calculating section 31, and themaximum bit rate at which the stream read section 2 reads out the AVstream 1a, employs the calculated readout time n' as a keyframetransmission interval. The stream readout position determining section31 then determines a keyframe to be read out next based on thecalculated keyframe transmission interval, the keyframe interval, andthe frame frequency.

Specifically, a playback speed PS is calculated as

    PS=(KI/FR)(NS+1)/n'                                        (6)

where n' is the continuous readout time of all the fixed-length blocksencompassed by a keyframe, KI is the keyframe interval, FR is the framefrequency, and NS is a keyframe skip number to a keyframe to be read outnext.

By using the above equations, the keyframe skip number NS is sodetermined that the playback speed PS becomes closest to the designatedplayback speed. If the keyframe interval KI is not constant, an averageof keyframe intervals of a stream, for instance, is employed as KI.

According to the second embodiment, by providing the stream transmissiontiming setting section 33 in the stream control section 60a, thekeyframe transmission interval is set longer than the maximum keyframereadout time during a high-speed playback. Therefore, even if thekeyframe data length varies in one stream, the transmission frameinterval is kept constant for an arbitrarily designated playback speed,thereby providing a high-speed playback that can be viewed morecomfortably.

EMBODIMENT 3

A video server apparatus according to a third embodiment of theinvention will be described below. FIG. 13 shows the configuration of astream control section 60b of a video server apparatus VS of thisembodiment. The other part of the video server apparatus VS is the sameas that of the first embodiment.

As shown in FIG. 13, the stream control section 60b has a stream readingposition determining section 61 for determining a keyframe that islocated in the vicinity of a playback start position designated by theterminal apparatus ST while cooperating with the access table managingsection 4, and for informing the stream reading section 2 of a readoutposition of the determined keyframe. The stream control section 60b alsohas a keyframe data length calculating section 62 for acquiring keyframedata lengths of at arbitrary intervals from the access table managingsection 4, a stream transmission timing setting section 63 for informingthe stream transmitting section 3 of transmission timing of eachkeyframe, and an access table page switching judgment section 64 forinstructing the access table managing section 4 to acquire, from thehard disk 1, keyframe addresses that are to be managed by the accesstable managing section 4.

FIG. 5B illustrates the operation of the access table managing section 4in this embodiment. In FIG. 5B, reference numerals 71-75 denote first tofifth pages of the access table 1b, numeral 76 denotes an nth page ofthe access table 1b, and character t denotes time that is taken to readout one page of the access table 1b at a constant bit rate.

The operation of the above-configured video server apparatus VS will bedescribed below (the stream control section 60b will be mainlydescribed). When the command receiving section 5 has received a playbackstart position, the stream readout position determining section 61determines a readout position of a keyframe that is located in thevicinity of the designated playback start position based on the receivedplayback start position and an address of the keyframe which is acquiredby the access table managing section 4 from the access table 1b that isstored in the hard disk 1. When the command receiving section 5 has notreceived any playback start position, the stream readout positiondetermining section 31 determines a readout position of a keyframe thatis designated to be read out next or continuous readout positions of theAV stream 1a. The determined information is sent to the stream readingsection 2.

If a playback speed received by the command receiving section 5 means adiscontinuous playback such as a high-speed playback, the stream readoutposition determining section 61 calculates a keyframe transmissioninterval based on all or an arbitrary number of keyframe data lengthsacquired from the access table 1b by the keyframe data lengthcalculating section 62 and the maximum bit rate at which the stream readsection 2 reads out the AV stream 1a, and informs the streamtransmission timing setting section 63 of the calculated keyframetransmission interval. The stream readout position determining section61 then determines a keyframe to be read out next that satisfies thedesignated playback speed based on the calculated keyframe transmissioninterval, the keyframe interval, and the frame frequency. If a playbackspeed received by the command receiving section 5 means a continuousplayback such as a normal playback, a slow playback, or the like, thestream readout position determining section 31 does not determine akeyframe to be read out next.

In the case of a high-speed playback, the stream transmission timingsetting section 63 instructs the stream transmitting section 3 totransmit a keyframe as read out by the stream reading section 2 at thekeyframe transmission interval that is designated by the stream readoutposition determining section 31, and informs the stream reading section2 of read timing of the next keyframe. The access table managing section4 reads out the access table 1b from the hard disk 1 on a page-by-pagebasis (one page corresponds to an arbitrary time or an arbitrary datalength). The access table page switching judgment section 64 compares aplayback start position associated with the current processing of thestream readout position determining section 61 with a positional rangeof pages that have been acquired by the access table managing section 4,and judges whether to acquire the next page of the access table 1b. Ajudgment result is sent to the access table managing section 4.

For example, when the stream readout position determining section 61 hasreceived a playback request, the access table page switching judgmentsection 64 instructs the access table managing section 4 to read out thefirst page 71 of the access table 1b. In response, the access tablemanaging section 4 reads out the first page 71 of the access table 1b.This reading takes time t. When the first page 71 has been acquired, thestream readout position determining section 61 starts its playbackprocessing. When the stream readout position determining section 61processes the head keyframe of the first page of the access table 1b,the access table page switching judgment section 64 instructs the accesstable managing section 4 to acquire the second page 72 of the accesstable 1b. The access table managing section 4 operates in the samemanner as in the above embodiments.

Thereafter, when the stream readout position determining section 61processes the head keyframe the second one of the pages being held bythe access table managing section 4, the access table managing section 4acquires a new page of the access table 1b. Thus, the access tablemanaging section 4 always retains 0, 1 or 2 access table pages.

According to the third embodiment, by providing the access table pageswitching judgment section 61 in the stream control section 60b, aplayback can be started upon acquisition of part of the access table 1b.Therefore, a response time from a playback request to a playback startcan be shortened.

In the invention, the manner of arranging contents of the access table1b in the access table managing section 4 and the manner of referencingthe access table 1b from the access table managing section 4, the datasize n of fixed-length blocks, the unit of dividing the access table 1b,and the frame configuration of the AV stream 1a are not limited to thosein the first to third embodiments. Although in the above embodiments thestorage medium is a hard disk, there may be used an optical disk, amagnetic tape, a semiconductor memory, arrayed disks such as a RAID, andan arrayed structures of other types of storage media. Although in theabove embodiments the storage subject is an AV stream, it may be a videostream or an audio stream. Although the above embodiments employ MPEG asthe coding scheme of the AV stream 1a, JPEG and other types of codingschemes may be employed. It is apparent that a more smooth high-speedplayback can be attained by making the bit rate at which the streamtransmitting section 3 transmits all or part of an AV stream 1a or thebit rate at which the stream reading section 2 reads out all or part ofthe AV stream 1a higher than that in a normal playback. Further, the AVstreams 1a may be stored in a physically distributed manner.

EMBODIMENT 4

FIG. 14 shows a video-on-demand system according to a fourth embodimentof the invention. As shown in FIG. 14, a video server apparatus of thissystem consists of a hard disk 101 for storing AV (audio-video) streams101a, a hard disk 102 for storing an access table 102b to be used inaccessing the AV streams 101a, a stream reading section 103 for readingout an AV stream 101a from the hard disk 101, a stream transmittingsection 104 for transmitting the read-out AV stream to a terminalapparatus, an access table managing section 105 for managing the accesstable 102b, a command receiving section 106 for receiving a command fromthe terminal apparatus, a stream control section 201 for controllingreading and transmission of the AV stream in accordance withinstructions from the terminal apparatus, a keyframe read/transmissionbit rate calculating section 202 for calculating a keyframe read bitrate, a keyframe transmission bit rate, and a keyframe data length whenonly keyframes of the AV stream are read out, and a control informationtransmitting section 203 for transmitting a keyframe playback to theterminal apparatus interval of a high-speed playback.

The terminal apparatus consists of an input control section 113 forgenerating a command in response to an input manipulation on an inputdevice 116, a command transmitting section 109 for transmitting thegenerated command to the video server apparatus, a stream receivingsection 110 for receiving an AV stream from the video server apparatus,a decoder section 112 for decoding the received AV stream, a controlinformation receiving section 204 for receiving information of akeyframe playback interval from the video server apparatus, and adecoder control section 205 for controlling the decoder section 112 sothat decoding is performed at the keyframe playback interval that isobtained from the control information receiving section 204 when theplayback mode that is obtained from the stream receiving section 110 ischanged to a high-speed playback mode. The input device 116 such as akeyboard or a mouse, a monitor 114 for displaying decoded images and aspeaker 115 for outputting decoded sound are connected to the terminalapparatus.

The operation of the above-configured system will be described below.

A signal that is input from the input device 116 is converted, by theinput control section 113 of the terminal apparatus, into a programdesignation command and commands indicating a playback start positionand a playback speed, which commands are transmitted from the commandtransmitting section 109 to the command receiving section 106 of thevideo server apparatus.

The stream control section 201 of the video server apparatus determinesan AV stream to be read out, keyframes to be read out, and their storagelocations based on the program information, the playback start position,and the playback speed that are received by the command receivingsection 106, and keyframe information that is managed by the accesstable managing section 105. The stream control section 201 informs thestream reading section 103 of the determined information, and instructsthe stream reading section 103 to read out the stream from the hard disk101.

If the playback speed means a high-speed playback, the keyframeread/transmission bit rate calculating section 102 calculates a keyframeaverage data length based on data lengths of the respective keyframes(i.e., I frames) of the AV stream that are recorded in the access table102b, calculates a read bit rate based on the keyframe average datalength, and a coding bit rate and a recording format (i.e., byte basis,fixed-length-block basis, or the like) of the AV stream that arerecorded in the access table 102b, and determines a transmission bitrate based on the number of terminal apparatuses currently communicatingwith the video server apparatus and other information.

In the case of a high-speed playback, the stream control section 201determines a keyframe playback interval and a keyframe readout intervalbased on the keyframe read bit rate, the key frame transmission bitrate, and the keyframe average data length that are obtained from thekeyframe read/transmission bit rate calculating section 202, a keyframeinterval that is obtained from the access table managing section 105,and the playback speed designated by the terminal apparatus. The streamcontrol section 201 informs the control information transmitting section203 of the keyframe playback interval. Further, the stream controlsection 201 informs the stream reading section 103 of the keyframereadout interval and the keyframe readout positions, and instructs thestream reading section 103 to read out the designated keyframes and toadd a playback mode indicating a change in playback speed to theread-out stream.

How the stream control section 201 determines the keyframe playbackinterval and the keyframe readout interval will be described later.

The control information transmitting section 203 transmits the keyframeplayback interval that has been determined by the stream control section201 to the terminal apparatus. On the other hand, the stream readingsection 103 reads out the designated keyframes of the AV stream 101a atthe designated read bit rate, and inserts or buries a playback mode intoa stream that consists only of the read-out keyframes. The streamtransmitting section 104 transmits the stream that is received from thestream reading section 103 to the stream receiving section 110 of theterminal apparatus. The stream control section 201 controls thetransmitting operation of the stream transmitting section 104 based onthe information of the transmission bit rate that is obtained from thekeyframe read/transmission bit rate calculating section 202.

In the terminal apparatus, the decoder control section 205 controls thedecoder section 112 so that decoding is performed in accordance with thekeyframe playback interval received by the control information receivingsection 204 by using, as a trigger, the playback mode extracted from thestream by the stream receiving section 110. The stream received by thestream receiving section 110 is decoded by the decoder section 112 andthen output from the monitor 114 as images. It is sufficient that thekeyframe playback interval be received at least once per change inplayback speed. In the case of a high-speed playback, the decodercontrol section 205 controls the decoder section 112 so that it does notdecode audio data.

The operation of a normal playback is the same as in the conventionalsystem.

As described above, in the system according to the fourth embodiment,the video server apparatus is provided with the stream control section201 for determining a keyframe readout interval and a keyframe playbackinterval that satisfy a playback speed that is designated by theterminal apparatus, as well as the control information transmittingsection 203 for transmitting the keyframe playback interval to theterminal apparatus. The terminal apparatus is provided with the controlinformation receiving section 204 for receiving the keyframe playbackinterval. This enables the video server apparatus to designate akeyframe playback interval that is to be used in the terminal apparatus,whereby a high-speed playback can be performed at an arbitrary playbackspeed that is requested by the terminal apparatus.

Next, a description will be made of how the keyframe playback intervaland the keyframe readout interval are determined by the stream controlsection 201. There are three methods, which will be described in fifthto seventh embodiments, respectively.

EMBODIMENT 5

In the first method for determining a keyframe playback interval and akeyframe readout interval that realize a designated playback speedmultiplication factor, the stream control section 201 of the videoserver apparatus divides a keyframe average data length that iscalculated by the keyframe read/transmission bit rate calculatingsection 202 by a smaller one of a keyframe read bit rate and a keyframetransmission bit rate, to thereby determine a minimum keyframe playbackinterval, that is, a minimum interval that allows complete transmissionof keyframes (keyframe data cannot be transmitted completely in a periodshorter than this interval).

Then, a shortest one of intervals consisting of the keyframe interval ofa stream and its integral multiples which intervals are longer than theminimum keyframe playback interval is selected, and is made a keyframeplayback interval. If a playback speed is designated by the terminalapparatus, a keyframe readout interval is calculated as a product of thekeyframe playback interval and the playback speed multiplication factor.

An example of the method for determining a keyframe playback intervaland a keyframe readout interval will be described with reference to aplayback speed table of FIG. 15, whose contents are the same as those ofthe table of FIG. 9. It is assumed that the minimum keyframe playbackinterval is 9. With the keyframe interval of 15, a smallest one amongmultiples of 15 which is larger than 9 is 15. Therefore, a keyframeplayback interval y is set at 15. Then, if a double-speed playback isdesignated by the terminal apparatus, a keyframe readout interval x iscalculated as 15×2=30. Thus, the stream reading section 103 reads outone keyframe per 30 frames; that is, frame numbers 1, 31, 61, . . . areread out.

Read-out keyframes are transmitted to the terminal apparatus. Adouble-speed playback is effected such that under the control of thedecoder control section 205, each keyframe is decoded 15-frames afterdisplay of the preceding keyframe as shown in FIG. 16A.

FIGS. 16B-16D show cases of triple-speed, quadruple-speed, and5-fold-speed playbacks, respectively.

This method can determine, by a simple procedure, a keyframe playbackinterval and a keyframe readout interval for realizing a playback speeddesignated by the terminal apparatus. Further, once keyframe playbackintervals are set for a certain stream, they will be effective forsubsequent high-speed playbacks of respective multiplication factors.Therefore, information of the keyframe playback interval can easily betransmitted to the terminal apparatus, and the keyframe playbackinterval can easily be controlled in the terminal apparatus.

EMBODIMENT 6

In the second method for determining a keyframe playback interval and akeyframe readout interval that realize a designated playback speedmultiplication factor, the stream control section 201 of the videoserver apparatus determines a minimum keyframe playback interval in thesame manner as in the fifth embodiment. Then, the stream control section201 selects a shortest one of intervals consisting of the keyframeinterval of a stream and its integral multiples which intervals arelonger than the minimum keyframe playback interval, and makes it a basickeyframe playback interval. If a playback speed is designated by theterminal apparatus, a basic keyframe readout interval is calculated as aproduct of the basic keyframe playback interval and the playback speedmultiplication factor.

Next, a search is made for such a combination of a keyframe playbackinterval y and a keyframe readout interval x that x/y coincides with thedesignated multiplication factor, where the keyframe playback interval yis longer than the minimum keyframe playback interval and shorter thanthe basic keyframe playback interval.

If an intended keyframe playback interval y and keyframe readoutinterval x are found by this search, they are employed as a keyframeplayback interval and a keyframe readout interval. If not found, thebasic keyframe playback interval and the basic keyframe readout intervalare employed as a keyframe playback interval and a keyframe readoutinterval.

An example of the method for determining a keyframe playback intervaland a keyframe readout interval will be described with reference to aplayback speed table of FIG. 17, whose contents are the same as those ofthe table of FIG. 9. It is assumed that the minimum keyframe playbackinterval is 9. With the keyframe interval of 15, a smallest one amongmultiples of 15 which is larger than 9 is 15. Therefore, a basickeyframe playback interval is set at 15. Then, if a double-speedplayback is designated by the terminal apparatus, a basic keyframereadout interval is calculated as 15×2=30.

Then, a search is made for such a combination of a keyframe playbackinterval y and a keyframe readout interval x that x/y is equal to 2,where y is in a range of 9 to 15 (region 141 in FIG. 17). Since no suchcombination is found in this case, the basic keyframe playback intervalof 15 is set as a keyframe playback interval and the basic readoutinterval of 30 is set as a keyframe readout interval.

If a triple-speed playback is designated by the terminal apparatus, abasic keyframe readout interval is calculated as 15×3=45. Then, a searchis made for such a combination of a keyframe playback interval y and akeyframe readout interval x that x/y is equal to 3, where y is in arange of 9 to 15. In this case, since a combination y=10 and x=30 isfound, 10 is set as a keyframe playback interval and 30 is set as akeyframe readout interval.

As is understood from the above examples, in the method of thisembodiment, the keyframe playback interval may vary with the playbackspeed.

In the case of a double-speed playback, as shown in FIG. 18A, the streamreading section 103 reads out one keyframe per 30 frames while beingcontrolled by the stream control section 201; that is, frame numbers 1,31, 61, . . . are read out. Read-out keyframes are transmitted to theterminal apparatus. A double-speed playback is effected such that underthe control of the decoder control section 205, each keyframe is decoded15-frames after display of the preceding keyframe as shown in FIG. 18A.

In the case of a triple-speed playback, as shown in FIG. 18B, the streamreading section 103 reads out one keyframe per 30 frames while beingcontrolled by the stream control section 201; that is, frame numbers 1,31, 61, . . . are read out. Read-out keyframes are transmitted to theterminal apparatus. A triple-speed playback is effected such that underthe control of the decoder control section 205, each keyframe is decoded10-frames after display of the preceding keyframe as shown in FIG. 18B.

FIGS. 18B-18D similarly show cases of quadruple-speed and 5-fold-speedplaybacks, respectively.

According to the method of this embodiment, since a keyframe playbackinterval and a keyframe readout interval for realizing a playback speeddesignated by the terminal apparatus are set as short as possible, themotion of high-speed playback images becomes more smooth and the imagequality is improved as much.

EMBODIMENT 7

The third method for determining a keyframe playback interval and akeyframe readout interval that realize a designated playback speedmultiplication factor is intended to cope with a playback request of anon-integral playback speed multiplication factor. In this method, akeyframe playback interval is determined for each keyframe received bythe terminal apparatus.

The stream control section 201 of the video server apparatus calculatesa minimum keyframe playback interval in the same manner as in the sixthembodiment. Where a non-integral number m is designated as a playbackspeed multiplication factor by the terminal apparatus, the streamcontrol section 201 then sets, as a basic keyframe readout interval, ashortest one of keyframe readout intervals that satisfy an inequality(keyframe readout interval)/(multiplication factor m)>(minimum keyframeplayback interval), that is, (keyframe readout interval)>(minimumkeyframe playback interval)×(multiplication factor m).

Now, a keyframe that is played back nthly (n is a natural number) iscalled "playback keyframe (n)," and an interval between playbackkeyframe (n) and playback keyframe (n+1) is called "keyframe playbackinterval (n)." The number of frames in a stored stream from the headframe to a frame immediately before playback keyframe (n+1) is equal to(basic keyframe readout interval)×n. To obtain a playback speedmultiplication factor that approximates the non-integral number m, theterminal apparatus may play back the above number of frames by setting,as a playback interval, an integer closest to (basic keyframe readoutinterval)×n÷(multiplication factor m). Therefore, keyframe playbackinterval (n) is equal to the integer closest to (basic keyframe readoutinterval)×n÷(multiplication factor m) minus an integer closest to (basickeyframe readout interval)×(n-1)÷(multiplication factor m). Keyframeplayback interval (n) is a variable value.

An example of determining a keyframe readout interval and keyframeplayback interval (n) will be described with reference to a playbackspeed table of FIG. 19, whose contents are the same as those of thetable of FIG. 9. It is assumed that the minimum keyframe playbackinterval is 9 and the keyframe interval is 15. A keyframe readoutinterval is determined in the following manner when a playback speedmultiplication factor 3.1 is designated by the terminal apparatus.

Keyframe playback intervals are calculated in order from onecorresponding to the minimum keyframe readout interval. Since keyframereadout intervals are multiples of the keyframe interval, first akeyframe corresponding to the minimum keyframe readout interval of 15 iscalculated as 15/3.1=4.83 . . . , which is smaller than the minimumkeyframe playback interval of 9. This value does not satisfy theinequality (keyframe readout interval)/(multiplication factorm)>(minimum keyframe playback interval).

Therefore, a keyframe playback interval corresponding to the secondsmallest keyframe readout interval of 30 is then calculated as30/3.1=9.67 . . . , which is larger than the minimum keyframe playbackinterval of 9. Thus, the keyframe readout interval of 30 is employed asa basic keyframe readout interval.

Next, keyframe playback interval (1) is calculated by subtracting aninteger 0 that is closest to (basic keyframe readout interval30)×(1-1)÷(multiplication factor 3.1)=0 from 10 that is obtained byrounding off 30/3.1=9.67 . . . to an integer. Thus, keyframe playbackinterval (1) is calculated as 10.

Keyframe playback interval (2) is calculated by subtracting an integer10 that is obtained by rounding off 30×(2-1)÷3.1=9.67 . . . to aninteger from 19 that is obtained by rounding off 30×2÷3.1=19.35 . . . toan integer. Thus, keyframe playback interval (2) is calculated as 9.

Similarly, keyframe playback interval (3) is calculated by subtractingan integer 10 that is obtained by rounding off 30×(3-1)÷3.1=19.35 . . .to an integer from 29 that is obtained by rounding off 30×3÷3.1=29.03 .. . to an integer. Thus, keyframe playback interval (3) is calculated as10. Subsequent keyframe playback intervals can be calculated in orderaccording to similar procedures. As is understood from the aboveexamples, the keyframe playback interval varies for each playbackkeyframe.

When a 3.1-fold-speed playback is designated by the terminal apparatus,the stream reading section 103 reads out one keyframe per 30 keyframesas shown in FIG. 20A while being controlled by the stream controlsection 201; that is, frame numbers 1, 31, 61, . . . are read out.Read-out keyframes are transmitted to the terminal apparatus, anddisplayed at the determined keyframe playback intervals under thecontrol of the decoder control section 205.

More specifically, after a decoded keyframe of frame number 1 isdisplayed, still display is effected for 9 frames and a keyframe offrame number 31 is displayed as an 11th display frame. Decoding of thekeyframe of frame number 31 is completed by its display. After thedisplay of the keyframe of frame number 31, still display is effectedfor 8 frames and a keyframe of frame number 61 is displayed as a 20thdisplay frame. Decoding of the keyframe of frame number 61 is completedby its display. After the display of the keyframe of frame number 61,still display is effected for 9 frames and a keyframe of frame number 91is displayed as a 30th display frame. Decoding of the keyframe of framenumber 91 is completed by its display. A 3.1-fold-speed playback isrealized as this type of playback processing is performed sequentially.

In the case of a 3.2-fold-speed playback, as shown in FIG. 20B, thestream reading section 103 reads out one keyframe per 30 keyframes; thatis, frame numbers 1, 31, 61, . . . are read out. In the terminalapparatus, the decoder control section 205 controls the decoder section112 to cause the following display sequence for the read-out keyframes:display of a keyframe of frame number 1, still display for 8 frames,display of a keyframe of frame number 31, still display of 9 frames,display of a keyframe of frame number 61, still display for 8 frames, .. . . Thus, a 3.2-fold-speed playback is realized.

FIGS. 20C and 20D show playback fame sequences of 3.3-fold-speed and3.4-fold-speed playbacks.

As described above, the method of this embodiment can realize anon-integral playback speed multiplication factor that is designated bythe terminal apparatus. This method can also be used in constructing,for instance, a jog shuttle in which the playback speed is variedgradually.

Although in the fifth to seventh embodiments the keyframe interval isassumed to be 15, the invention is not limited to such a case. Thekeyframe interval may even be varied. The playback speed multiplicationfactor is not limited to the values used in the respective embodiments,but may be a negative value which means a backward high-speed playback.Although in the seventh embodiment a calculated number is rounded off toan integer in calculating keyframe playback interval (n), the inventionis not limited to such a case. Further, although in the respectiveembodiments the still display of a playback keyframe is controlled byusing the keyframe playback interval, it may be controlled by using theabsolute frame number as counted from the playback start keyframe orsome other keyframe.

EMBODIMENT 8

An eighth embodiment of the invention is a modification of the videoserver apparatus and the terminal apparatus of the video-on-demandsystem according to the fourth embodiment.

In the video server apparatus of this system, as shown in FIG. 21,information of a keyframe playback interval that is set in the videoserver apparatus is buried into a stream and transmitted to the terminalapparatus together with the stream. Therefore, the control informationtransmitting section 203 and the control information receiving section204 that are respectively provided in the video server apparatus and theterminal apparatus in the fourth embodiment (see FIG. 14) are notprovided in this embodiment. The eighth embodiment is the same as thefourth embodiment in the other part of the configuration.

In this system, a signal that is input from the input device 116 isconverted, by the input control section 113 of the terminal apparatus,into a program designation command and commands indicating a playbackstart position and a playback speed, which commands are transmitted fromthe command transmitting section 109 to the command receiving section106 of the video server apparatus.

The stream control section 301 of the video server apparatus determinesan AV stream to be read out, keyframes to be read out, and their storagelocations based on the program information, the playback start position,and the playback speed that are received by the command receivingsection 106, and keyframe information that is managed by the accesstable managing section 105. The stream control section 301 informs thestream reading section 306 of the determined information, and instructsthe stream reading section 306 to read out the stream from the hard disk101.

If the playback speed means a high-speed playback, a stream controlsection 301 determines a keyframe playback interval and a keyframereadout interval based on a keyframe read bit rate, a key frametransmission bit rate, and a keyframe average data length that areobtained from the keyframe read/transmission bit rate calculatingsection 202, a keyframe interval that is obtained from the access tablemanaging section 105, and the playback speed designated by the terminalapparatus. The stream control section 301 informs the stream readingsection 306 of the keyframe readout interval, the keyframe readoutpositions, the keyframe read bit rate, and the keyframe playbackinterval, and instructs the stream reading section 306 to read out thedesignated keyframes and to insert or bury the keyframe playbackinterval into the read-out stream.

The stream reading section 306 reads out the designated keyframes of theAV stream 101a at the designated read bit rate, and inserts or buriesthe keyframe playback interval into a stream that consists only of theread-out keyframes. The stream transmitting section 104 transmits thestream that is received from the stream reading section 306 to a streamreceiving section 307 of the terminal apparatus.

In the terminal apparatus, a decoder control section 305 controls thestream decoding of the decoder section 112 based on the information ofthe keyframe playback interval that is extracted from the stream by thestream receiving section 307. The stream decoded by the decoder section112 is output from the monitor 114 as images.

Even in the case where the keyframe playback interval varies for thesame playback speed as in the case of the seventh embodiment, thedecoder control section 305 of this terminal apparatus can easilycontrol the decoder section 112 operating in accordance with the varyingkeyframe playback interval, thereby enabling fine playback speedcontrol.

The operation of a normal playback is the same as in the conventionalsystem.

As described above, the system of the eighth embodiment can easily copewith a case where the terminal apparatus requests a precise high-speedplayback such as a playback of a non-integral playback speedmultiplication factor. Thus, the system can realize a high-speedplayback that satisfies a request from the terminal apparatus.

EMBODIMENT 9

In a video-on-demand system according to a ninth embodiment of theinvention, a keyframe playback interval and a keyframe readout intervalare set on the terminal apparatus side.

As shown in FIG. 22, a video server apparatus of this system consists ofa hard disk 101 for storing AV (audio-video) streams 101a, a hard disk102 for storing an access table 102b to be used in accessing the AVstreams 101a, a stream reading section 406 for reading out an AV stream101a from the hard disk 101, a stream transmitting section 104 fortransmitting the read-out AV stream to a terminal apparatus, and astream readout control section 408 for controlling reading andtransmission of the AV stream.

The terminal apparatus consists of an input control section 113 forgenerating a command in response to an input manipulation on an inputdevice 116, an access table managing section 105 for reading outinformation of the access table 102b that is stored in the video serverapparatus, a keyframe read/transmission bit rate calculating section 202for calculating a keyframe read bit rate, a keyframe transmission bitrate, and a keyframe data length based on the information read out bythe access table managing section 105, a stream control section 401 fordetermining a keyframe playback interval and a keyframe readoutinterval, a stream receiving section 407 for receiving an AV streamtransmitted from the video server apparatus, a decoder section 112 fordecoding the received AV stream, and a decoder control section 405 forcontrolling the decoder section 112. The input device 116 such as akeyboard or a mouse, a monitor 114 for displaying decoded images and aspeaker 115 for outputting decoded sound are connected to the terminalapparatus.

The operation of the above-configured system will be described below.

A signal that is input from the input device 116 is converted, by theinput control section 113 of the terminal apparatus, into a programdesignation information and information indicating a playback startposition and a playback speed, which information is input to the streamcontrol section 401. The stream control section 401 instructs, based onthe playback start position of the designated program and the playbackspeed, the access table managing section 105 to acquire keyframeinformation corresponding to the designated program from the accesstable 102b that is stored in the video server apparatus.

Then, the stream control section 401 determines an AV stream to be readout, keyframes to be read out, and their storage locations based on theplayback start position and the playback speed of the designatedprogram, and the keyframe information held by the access table managingsection 105. The stream control section 401 informs the stream readoutcontrol section 408 of the video server apparatus of the determinedinformation, and instructs the stream readout control section 408 toread out the stream from the hard disk 101.

If the playback speed means a high-speed playback, the keyframeread/transmission bit rate calculating section 202 calculates a keyframeaverage data length based on data lengths of the respective keyframes ofthe AV stream that are obtained from the access table managing section105, and also calculates a keyframe read bit rate and a keyframetransmission bit rate based on the keyframe average data length, acoding bit rate and other information.

The stream control section 401 determines a keyframe playback intervaland a keyframe readout interval based on the keyframe average datalength, the keyframe read bit rate, the key frame transmission bit rate,a keyframe interval that is obtained from the access table managingsection 105, and the designated playback speed. The stream controlsection 401 informs the stream readout control section 408 of the videoserver apparatus of the keyframe readout interval, the keyframe readoutpositions, and the keyframe read bit rate. Further, the stream controlsection 401 informs the decoder control section 405 of the determinedkeyframe playback interval.

The stream readout control section 408 of the video server apparatuscontrols the stream reading section 406 so that it reads out thedesignated keyframes of the AV stream 101a at the designated read bitrate. A read-out stream is sent to the stream transmitting section 104,which transmits the received stream to the stream receiving section 407of the terminal apparatus.

In the terminal apparatus, the decoder section 112 decodes the streamreceived by the stream receiving section 407. The decoder controlsection 405 controls the decoder section 112 so that the decoding isperformed in accordance with the keyframe playback interval obtainedfrom the stream control section 401. A video signal produced by thedecoding of the decoder section 112 is output from the monitor 114 asimages.

Even when the keyframe playback interval varies for the same playbackspeed, the decoder control section 405 of the terminal apparatus cancontrol the decoding of the decoder section 112 while receiving theinformation of the keyframe playback interval directly from the streamcontrol section 401. Therefore, this system can properly cope withfrequent changes in keyframe playback interval.

As described above, in the system of this embodiment, the terminalapparatus is provided with the stream control section 401, the keyframeread/transmission bit rate calculating section 202, and the access tablemanaging section 105. As a result, a keyframe playback interval can bedesignated on the terminal apparatus side, and hence fine control of ahigh-speed playback can be performed easily.

Although in the fourth to ninth embodiments a keyframe playback intervaland other parameters are calculated by using a keyframe average datalength, they may be calculated by using a keyframe maximum data lengthinstead of the keyframe average data length. Although in thoseembodiments the AV streams 101a and the access table 102b are stored inthe separate hard disks 101 and 102, it is apparent that they may bestored in the same storage medium. Instead of the AV streams 101a, thesubject streams may be video streams, audio streams, or some other typeof streams. Although the MPEG scheme is used to compress moving images,the invention is not limited to such a case. Further, the terminalapparatus may be adapted to decode audio data to allow sound output fromthe speaker even in a high-speed playback.

The connection between the video server apparatus and the terminalapparatus may take a variety of form. For example, a plurality ofterminal apparatuses may be connected to a single video serverapparatus, a plurality of terminal apparatuses may be connected to aplurality of video server apparatuses, or a single terminal apparatusmay be connected to a single video server apparatus.

In the eighth embodiment, a keyframe playback interval may be insertedor buried into a bit stream such that it is converted into timeinformation indicating time points at which subject keyframes aredecoded and played back and the time information is described in DTS(decoding time stamp) fields and PTS (presentation time stamp) fieldsthat are specified in the international standards MPEG1 (moving picturecoding experts group phase 1) and MPEG2 (moving picture coding expertsgroup phase 2). Alternatively, it may be inserted or buried as timeinformation in a manner not specified in MPEG1 or MPEG2. Further, in theeighth embodiment, a playback mode may also be inserted or buried into astream as in the case of the fourth embodiment.

What is claimed is:
 1. A video server apparatus comprising:stream storage means for storing compressed and coded streams; stream reading means for reading out one of the streams which is designated by a terminal apparatus; stream transmitting means for transmitting the read-out stream to the terminal apparatus; access table managing means for managing addresses of keyframes of the stream; keyframe data length calculating means for acquiring data lengths of the keyframes; and stream readout position determining means for determining a first keyframe located in the vicinity of a playback start position that is designated by the terminal apparatus based on the addresses managed by the access table managing means, informing the stream reading means of a readout position of the first keyframe, selecting keyframes to be extracted from the stream so as to satisfy an arbitrary playback speed that is designated by the terminal apparatus based on a bit rate at which the stream reading means reads out the stream, a coding bit rate and a keyframe interval of the stream, and the keyframe data lengths acquired by the keyframe data length calculating means, and informing the stream reading means of the selected keyframes. 